A legged robot has a plurality of leg portions and walks while swinging the respective leg portions. A legged robot which walks while balancing on two legs like a human is called a biped walking robot. A legged robot which walks on four legs like an animal is called a quadrupedal walking robot. The number of legs is typically two or four, but any number of legs more than one may be provided.
Each of the leg portions is formed by connecting links corresponding to a thigh portion, a lower leg portion, and a foot portion in order from a trunk portion with a hip joint, a knee joint, and an ankle joint interposed therebetween. Each of the links corresponding to the thigh portion, the lower leg portion, and the foot portion is connected to each of the joints to be rotatable about a pitch axis extending sideways relative to the legged robot. Each of the joints includes a motor (actuator) for rotating the link (see Patent Literature 1, for example). The motors output appropriate drive forces to control rotation angles of the links and, as a result, the legged robot can swing the leg portions forward and backward relative to the trunk portion. Here, the hip joints refer to joints connecting the trunk portion and the leg portions and joints connecting a trunk portion and front legs of the quadrupedal walking robot are also included in the hip joints. The motors are devices for converting energy of electricity, gasoline, or the like into mechanical movements and are typically electric motors or engines.